Technological Field
The present invention relates to an image forming system including a plurality of image forming apparatuses which, after output of an image on paper, inspect the image, and a non-transitory recording medium storing a computer-readable program.
Description of the Related Art
In the past, a technique has been known in which the paper on which an image has been printed by an image forming apparatus according to a job is read by a reader installed at the back side of the image forming apparatus and compared against data for abnormal image detection as a reference to determine whether or not the image is abnormal and then subsequent printing is stopped or reprinting is done. This kind of technique is called spoilage detection.
Generally, an image is determined as abnormal in the following cases: the image obtained by reading an output image formed on paper (hereinafter called “read output image”) is stained; the printed content is incorrect; the paper and output image are considerably misaligned, and the like. One method for determining whether or not the image is abnormal is as follows: the density or brightness of a given pixel and a peripheral pixel in each of the original image of the job and the read output image is calculated and if the difference exceeds the threshold, the image is determined as abnormal.
FIG. 1 is an explanatory drawing which shows an example of an original image and a read output image. The arrows denote the scanning direction (direction of difference comparison). In the original image 200 shown in FIG. 1, circular road signs 201 and 202 are arranged vertically. In the image forming apparatus, the original image 200 and the read output image 210 after job output (output image on paper S) are compared and if there is a difference between corresponding pixels, the output image is determined as an abnormal image which contains a line 213 and stain 214.
However, when comparison is made only between the read output image and the original image, the image may be determined as an abnormal image if the printing position is not aligned with the paper edge or if the image magnification ratio has changed due to shrinkage of the paper, etc. The reason for this is that even when the read output image has no problem as an output result, the read output image is not exactly the same as the original image.
FIG. 2 is an explanatory drawing which shows an example of an original image and a scaled read output image.
Actually, the magnification ratio changes due to shrinkage of paper caused by thermal fixing or the like and the read output image is often not exactly the same as the original image. In the example shown in FIG. 2, L denotes the length from the upper end of the road sign 201 to the lower end of the road sign 202 in the original image 200 and L′ denotes the length from the upper end of the road sign 201 to the lower end of the road sign 202 in the read output image 210. As a result of shrinkage of the paper S, the read output image 210 is smaller than the original image 200 (L′<L), but it has neither stain nor line (the output has no problem). However, since the read output image 210 has an area which is different from the original image 200 (area enclosed by the chain line), it may be mistakenly determined as being stained.
For this reason, the read output image, the original image, and data for abnormal image detection (hereinafter called “reference data”) are used in order to determine whether or not the output image is abnormal. The reference data is image data used in order to prevent false abnormal image detection and generally an edge image which contains data representing an edge position in an image is used as reference data.
FIG. 3 is an explanatory drawing which shows an example of an original image and an edge image (reference data).
In comparison between images, the possibility of false detection caused by a difference in magnification ratio or a minor misalignment can be reduced by setting a slightly high threshold for an image edge position to determine abnormality. For example, when comparison is made between the original image 200 and the read output image 210, if the threshold for an edge position 232 in an extracted area 231 including the lower end of the road sign 202 in the edge image 230 is set to a slightly high value, misalignment between the lower end of the road sign 202 in the original image 200 and the lower end of the road sign 202 in the read output image 210 is not mistakenly detected as abnormal.
As shown in FIG. 3, the edge image 230 is created by the image forming apparatus receiving the original image 200 and checking each target pixel in the original image 200 and pixels around the target pixel. For this reason, if the original image 200 contains many edges, the hardware resources of the image forming apparatus (CPU, etc.) would be considerably consumed to generate reference data, resulting in long processing time.
FIG. 4 is a flowchart which shows the sequence from receipt of a job to image abnormality inspection in an ordinary image forming apparatus to image abnormality inspection.
When the image forming apparatus receives a job (original image) (S1), the apparatus determines whether or not the job is an abnormal image detection job which involves image abnormality inspection after job output (S2) and if the job is an abnormal image detection job (YES at S2), reference data is generated (S3). Then, the image forming apparatus performs output of the job (S4), and an image abnormality inspection is conducted on the output image formed on paper (S5), and processing after abnormal image detection is carried out as necessary. For example, if an abnormal image is detected, notice of image abnormality is given to the user or output of the job is stopped. On the other hand, if the received job is not an abnormal image detection job (NO at S2), the image forming apparatus performs output of the job (S4) and ends the processing sequence without conducting an image abnormality inspection at Step S5.
If image abnormality inspection is conducted using reference data, it is desirable that the image forming apparatus can generate reference data efficiently because image abnormality inspection cannot be started before generation of reference data is completed.
Patent Literature 1 (JP-A-2017-90444) discloses a technique as an image inspection method using such an edge image, in which an edge is extracted from an original image and in comparison between the read output image and the original image, a decision is made as to whether or not the image is a spoilage (abnormal image), according to the amount of change in the density of the vicinity of the edge. If the density difference in the edge is equal to or more than the threshold, the image is determined as an abnormal image.